Thulium and/or holmium doped silicated glasses for two micron lasers

ABSTRACT

A laser glass fiber with a core of the fiber composition, comprising a silicate glass host, one or more glass network modifiers, one or more glass network intermediators, and Thulium ions, Holmium ions, or a combination of Thulium ions and Holmium ions. The fiber emits laser light from 1.7 micron to 2.2 micron.

GOVERNMENT RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Grant No.FA8650-07-M-5412.

FIELD OF THE INVENTION

This invention relates to glasses and fibers for laser applications.More specifically, this invention is directed to Thulium-doped,Holmium-doped, and Thulium/Holmium co-doped, silicate glasses for near 2micron fiber lasers with high quantum efficiency.

BACKGROUND OF THE INVENTION

Near 2 micron fiber lasers are of great interest because of thepotential possibility of combining high output power and retina safetytogether, which are needed for a wide variety of commercial and militaryapplications including materials process, remote sensing, andbio-medical application. Due to the stronger absorption of manymaterials containing water molecules and organic materials at 2 micronthan at 1 micron, less power of 2 micron fiber laser is needed comparedto 1 micron fiber laser in order to achieve the same effect. Moreimportantly, 2 micron is classified as retina safe wavelength(frequently called as eye-safe laser), which is much safer than 1 micronlaser. Up to now, majority of the high power fiber laser developmentfocus on 1 micron Yb³⁺-doped fiber laser.

SUMMARY OF THE INVENTION

Considering the inherent critical drawbacks associated with silica andgermanate glass fibers, Applicants have prepared highly Tm³⁺-doped,Ho³⁺-doped, and Tm³⁺/Ho³⁺-doped, silicate glass fiber for 2 micron fiberlaser application. By “silicate glass,” Applicants mean multi-componentglass with SiO₂ as the glass network former. In contrast to silicaglass, silicate glass contains glass network modifiers such as alkaliions and alkaline metal ions, and glass network intermediates such asaluminum oxide and boron oxide in addition to glass network former ofSiO₂. In most cases, the content of SiO₂ is not higher than 80 molepercent in silicate glasses.

Applicants have found that high concentration of rare-earth oxides canbe doped into silicate glasses without the so-called ion-clusters mainlybecause of the glass network modifiers. The glass network modifiers,such as sodium ions, potassium ions, barium ions, and calcium ions,break the well-defined glass network of silica, thereby producing sitesfor rare-earth ions. Applicants have found that silicate glass fiberexhibits numerous advantages over silica fiber, germanate glass fiberand other multi-component glass fibers as the host for highly efficientand high power fiber laser near 2 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a reading of the followingdetailed description taken in conjunction with the drawings in whichlike reference designators are used to designate like elements, and inwhich:

FIG. 1 illustrates absorption spectra of Tm³⁺-doped silicate glasseswith different doping concentrations;

FIG. 2 illustrates thermal expansion curves of Tm³⁺-doped silicate glassand undoped cladding glass;

FIG. 3 illustrates fiber laser performance;

FIG. 4 shows a cross section view of single cladding Tm³⁺-doped silicateglass fiber;

FIG. 5 shows a cross section view of on off-center single claddingTm³⁺-doped silicate glass fiber;

FIG. 6 shows a cross section view of doubling cladding Tm³⁺-dopedsilicate glass fiber;

FIG. 7 shows a cross section view of on off-center double claddingTm³⁺-doped silicate glass fiber; and

FIG. 8 is a block diagram showing Applicants' apparatus used for fiberlaser characterization.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbersrepresent the same or similar elements. Reference throughout thisspecification to “one embodiment,” “an embodiment,” or similar languagemeans that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment.

The described features, structures, or characteristics of the inventionmay be combined in any suitable manner in one or more embodiments. Inthe following description, numerous specific details are recited toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventionmay be practiced without one or more of the specific details, or withother methods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

Near 2 micron fiber lasers can be generated from Tm³⁺-doped, Ho³⁺-dopedand Tm³⁺/Ho³⁺-co-doped fibers. The laser wavelengths can vary from 1.8micron to 2.2 micron, which is generally called 2 micron fiber laser.Tm³⁺-doped and Tm³⁺/Ho³⁺-co-doped fibers can be used to generate near 2micron fiber lasers because diode lasers near 800 nm can be used as apump source. In some cases pump lasers from 1.5 micron to 1.9 micron areused to excite the active rare-earth ions from the ground state to thelasing state, which is called in-band pumping. When a near 800 nm laseris used as pump source, the quantum efficiency can be close to 200%because of the so-called cross-relaxation process of Tm³⁺ ions.

Tm³⁺ cross-relaxation is a non-radiative process in which an excitedTm³⁺ in the ³H₄ level decays to the ³F₄ level and a neighboringground-state Tm³⁺ ion is excited to the ³F₄ level, accompanied by theemission of phonons when the Tm³⁺ doping concentration is sufficientlyhigh. Tm³⁺ doped crystals the probability of Tm³⁺ cross relaxation isnegligible for concentration less than about 2 weight percent butapproaches unity for concentrations greater than about 5 weight percent.

The cross-relaxation process was observed in Tm³⁺-doped silica fiberwhen the doping concentration reached 2.2 weight percent, resulting in aquantum efficiency of 120%. Cross-relaxation was also realized ingermanate glasses with 4 weight percent Tm³⁺ doping concentration, wherea quantum efficiency of 170% was demonstrated. Highly efficient fiberlasers were demonstrated in both silica fiber laser and germanate glassfiber by taking advantage the cross-relaxation of Tm³⁺ ions. By usingTm³⁺-doped germanate glass fiber 2 μm fiber laser with more than 100 Woutput power and 68% slope efficiency was demonstrated.

However, both Tm³⁺-doped silica fiber and Tm³⁺-doped germanate glassfiber suffer many problems for practical fiber laser applications. ForTm³⁺-doped silica fiber, the doping concentration of Tm³⁺ ions isrestricted to around 2.2 weight percent of Tm₂O₃, which limits thebenefit of cross-relaxation of Tm³⁺ ions. As a result, the efficiency isrelatively low. The limited cross-relaxation also results in morevisible and UV emission via upconversion, which accelerate thephotodarkening of Tm³⁺-doped silica fiber.

When photodarkening occurs, the laser efficiency decreases, andsometimes the laser operation even stops. Tm³⁺-doped germanate glassfiber also suffers photodarkening problems. The major component ofgermanate glasses is germanium oxide. Germanium oxide can be doped intosilica glass fiber to produce photosensitivity of silica fiber.Germanate glass is also photosensitive. Therefore, both Tm³⁺-dopedsilica fiber and germanate glass fiber suffer photo-darkening problem,which significantly affects the fiber laser performance, especially forlong term performance. Another problem associated with germanate glassfiber is that GeO₂ is very expensive.

It is well known that the doping concentration of rare-earth ions insilica fiber is limited due to the intrinsic glass network structure.Various approaches are developed to increase the doping concentrationincluding co-doping with Al₂O₃, B₂O₃, and P₂O₅ and using nano-particles.The highest doping level is limited to 1 or 2 weight percent ofrare-earth oxide, which is still far away from 4 to 6 weight percentrequired for efficient cross-relaxation energy transfer of Tm³⁺ ions.High Tm³⁺ doping concentrations can be achieved in silicate glasses dueto its less defined glass network, which enables the maximum benefit ofcross-relaxation energy transfer. Quantum efficiency near 200% can beachieved when Tm³⁺-doped fiber laser is pumped with near 800 nm laserdiodes. Such high quantum efficiency will results in a high slopeefficiency and relatively small amount of heat.

Because of the limited doping concentration of Tm³⁺ ions in silicafiber, the cross-relaxation process is limited, which results in morevisible and UV emission via upconversion. UV emission accelerates thephotodarkening of Tm³⁺-doped silica fiber. Photo-darkening is anexisting issue for high power Yb³⁺-doped silica fiber laser where almostno visible light is produced in the active fiber because of the simpletwo level energy system of Yb³⁺ ions.

A significant advantage of silicate glass fiber over germanate glassfiber is the stronger resistance to photo-darkening. The major componentof germanate glasses is germanium oxide. Germanium oxide is commonlydoped into silica glass fiber to produce photosensitivity of silicafiber. Obviously germanate glass is photosensitive.

Silicate glasses and germanate glasses comprise different glass networkformers. The bond strength of Si—O is stronger than that of Ge—O.Logically the mechanical strength of silicate glasses typically isstronger than germanate glasses, and the coefficient of thermalexpansion of silicate glasses is smaller. The smaller the coefficient ofthermal expansion the higher the thermal shock resistance. A higherthermal shock resistance and a stronger mechanical strength yield ahigher pump heat induced damage threshold and laser induced damagethreshold, which are critical in order to achieve high fiber laserpower.

Although silica fiber will not be used for laser generation, Passivessilica fiber is the most suitable fiber for laser delivery and probablyalso for gratings to form the laser cavity. Silica fiber has relativelylow loss (˜0.1 dB/m) at 2 μm as laser delivery fiber because typically afew meters of fiber are sufficient. Rare-earth doped Ssilicate glassfiber can be easily fusion spliced with silica fiber.

The fabrication cost of silicate glasses and germanate glasses is verysimilar. But the price of GeO₂ is one order of magnitude higher thanSiO₂, which makes germanate glasses extremely expensive.

Other multi-component glasses such as phosphate glasses, borate glasses,tellurite glasses, and fluoride glasses are not suitable as host of Tm³⁺ions for high power 2 micron laser generation. Phosphate glasses andborate glasses are not suitable because of their higher phonon energy.Phosphate glasses and borate glasses exhibit phonon energy of ˜1300cm⁻¹, and ˜1350 cm⁻¹, respectively. Due to the narrow energy gap betweenthe ³F₄-³H₆ transition of Tm³⁺ ions, quantum efficiency can benoticeably reduced through multi-phonon relaxation process when the hostmaterial has high phonon energy. The maximum phonon energy of silica andsilicate glass is almost identical, ˜1100 cm⁻¹, while the overall phononenergy of silicate glass is slightly smaller. Germanate glass exhibitssmall phonon energy of ˜900 cm⁻¹. In fact, both tellurite glass andfluoride glasses have much smaller phonon energy of ˜750 cm⁻¹, and ˜500cm⁻¹, respectively, which could be advantageous as host for Tm³⁺ ionsfor 2 μm laser generation. Unfortunately, their weak mechanicalstrength, lower thermal shock resistance, and poor chemical durabilityprevent their use for high power fiber laser applications.

Applicants designed glass compositions, and fabricated a series ofsilicate glasses, which exhibit good rare-earth solubility and thermalproperties. The strong Si—O bonds which form the glass host matrix canbe perturbed using one or more Network Modifiers (“MO”). Generallyspeaking, Network Modifiers affect, inter alia, the thermal expansion,hardness, chemical durability, density, surface tension, and refractiveindex, of a pure SiO₂ glass. In certain embodiments, Applicants'silicate glass SiO₂ glass host is modified using one or more MOmaterials selected BaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O, Li₂O, Y₂O₃, orcombinations thereof, from 5 weight percent to 40 weight percent.

In certain embodiments, Applicants' laser glass composition comprisesone or more glass network intermediators (XO). A glass networkintermediator modifies the host glass network, thereby creatingadditional dopant sites. In certain embodiments, the one or more glassnetwork intermediators bridge some of the bonds in the network therebyincreasing the network's strength and chemical durability withoutraising the melting temperature appreciably. In certain embodiments,Applicants' silicate glass SiO₂ glass host is modified using one or moreXO materials selected from Al₂O₃, B₂O₃, La₂O₃, or combinations thereof,from 0.5 weight percent to 40 weight percent. Table I recitesApplicants' glass compositions.

TABLE 1 Glass compositions in mole percent Glass No. SiO₂ Al₂O₃ Li₂ONa₂O CaO BaO S—Tm-1 55 5 7.5 7.5 12.5 12.5 S—Tm-2 55 5 10 10 10 10S—Tm-3 55 2.5 7.5 7.5 13.75 13.75 S—Tm-4 55 2.5 8.75 8.75 12.5 12.5

Good rare-earth solubility is needed because, in certain embodiments,the doping concentration of Tm₂O₃ is between about 4 weight percent toabout 7 weight percent. Glass has to exhibit excellent thermalproperties because during fiber drawing process fiber preform will beheated up to glass softening temperature, which is around 200° C. abovethe glass transition temperature. No crystallization should occur duringfiber drawing process. Otherwise, scattering loss will be introduced inthe fiber.

Applicants utilized starting materials comprising a purity greater thanabout 99.99%. Glasses were melted in a platinum crucible in a resistanceheated furnace. Typically, chemicals were loaded at 1350° C., and heatedto 1500° C. after melting. Glass liquid was kept at around 1500° C. inliquid nitrogen protected environment in order to remove OH⁻ groups fromthe glass. Then the glass was cast into a preheated aluminum mold.

After the glass solidifies, it is moved into an annealing oven forannealing process to remove the cast induced stress. Glass samples forthermal properties characterization, refractive index measurement, andabsorption measurement were fabricated.

Applicants fabricated silicate glasses with doping concentrations of 2weight percent, 3 weight percent, 4 weight percent, 5 weight percent,and 6 weight percent of Tm₂O₃. FIG. 1 illustrates absorption spectra 100comprising spectrum 110 for a 2 weight percent Tm₂O₃ doping, andspectrum 120 for a 6 weight percent Tm₂O₃ doping.

Undoped cladding glasses were also fabricated. As those skilled in theart will appreciate, cladding glasses must comprise a similar thermalexpansion coefficient with respect to the doped core glass to ensure lowmechanical and thermal stress in the fiber, and a lower refractive indexcompared to the core glass to form waveguide.

FIG. 2 illustrates thermal expansion curves of both Tm³⁺-doped silicateglass and undoped silicate glass. The difference of the thermalexpansion coefficients between the core glass and the cladding glass isless than 10%.

Fiber preforms were formed with doped core glass rod and undopedcladding glass tubes. Doped core glass rods were drilled from a bulkcore glass using diamond core drill. The barrel of the rod is polishedto a high surface quality. Undoped cladding glass tubes were drilledfrom cladding glasses. Both inside and outside surfaces of the glasstubes are polished to a high surface quality. Tm³⁺-doped silicate glassfibers were pulled in our own fiber drawing tower.

FIG. 4 shows a fiber preform 400 comprising cladding 410 and core 420.In certain embodiments, the cladding diameter was about 229 microns, andthe core diameter was about 41.5 microns.

FIG. 5 shows a fiber preform 500 comprising cladding 510 and off-centercore 520. In certain embodiments, the cladding diameter was about 216microns, and the core diameter was about 18.5 microns.

FIG. 6 shows a fiber preform 600 comprising first cladding 610, secondcladding 620, and core 630. In certain embodiments, the first claddingdiameter was about 245 microns, the second cladding diameter was about210 microns, and the core diameter was about 21.5 microns.

FIG. 7 shows a fiber preform 700 comprising first cladding 710, secondcladding 720, and off-center core 730. In certain embodiments, the firstcladding diameter was about 231 microns, the second cladding diameterwas about 195.5 microns, and the core diameter was about 20.5 microns.

FIG. 8 illustrates apparatus 800 which was used for fiber lasercharacterization. Fiber pigtailed multi-mode 790 nm diode laser 810 wasused as pump source. The core of the pump laser delivery fiber is 200micron. The delivery fiber of the pump laser is fusion spliced with asilica fiber 820 with dielectric coating. Dielectric mirror was coatedat the end of silica fiber with core and cladding diameters of 200microns and 240 microns, respectively. The dielectric coating exhibitshigh reflectivity at 1.9 micron and high transmission at 790 nm. Nocoating is used for output coupler. Fresnel reflection of approximately5% from the Tm³⁺-doped silicate glass fiber was used to form the fiberlaser cavity.

FIG. 3 shows the fiber laser test results using apparatus 800 (FIG. 8)and F—Tm-6 fiber with different fiber lengths. The cross section view ofF—Tm-6 fiber was shown in FIG. 7. Curve 310 was obtained using a 20 cmfiber length. Curve 320 was obtained using a 30 cm fiber length. Curve340 was obtained using a 30 cm fiber length.

A slope efficiency of 38% was achieved in F—Tm-6 fiber with 30-cm fiberlength. It should be noted that the slope efficiency depends heavily onthe reflectivity of the output coupler. Here the reflectivity of theoutput coupler is fixed because we simply use the Ffresnel reflection ofthe output end of the fiber. The slope efficiency can be significantlyimproved by optimizing output coupler.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

We claim:
 1. A laser fiber to emit laser light from 1.7 micron to 2.2micron, comprising: a silicate laser glass comprising: a network formerof SiO₂ from about 30 weight percent to about 90 weight percent; a glassnetwork modifier (MO) selected from BaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O,Li₂O, Y₂O₃, or combinations thereof, from 5 weight percent to 40 weightpercent; and a glass network intermediator (XO) selected from Al₂O₃,B₂O₃, La₂O₃, or combinations thereof, from 0.5 weight percent to 40weight percent; Holmium ions; and Thulium ions.
 2. The laser fiber ofclaim 1, wherein: said Thulium ions are introduced into said glass hostas Thulium oxide; and said Holmium ions are introduced into said glasshost as Holmium oxide.
 3. The laser fiber of claim 2, comprising:Thulium oxide from about 2 weight percent to about 15 weight percent;and Holmium oxide from about 0.1 weight percent to about 3 weightpercent.
 4. The laser fiber of claim 3, comprising: Thulium oxide fromabout 3 weight percent to about 10 weight percent; Holmium oxide fromabout 0.2 weight percent to about 2 weight percent.
 5. The laser fiberof claim 4, comprising: SiO₂ from about 40 weight percent to about 80weight percent; said glass network modifier from about 10 weight percentto about 40 weight percent; said glass network intermediator from about1 weight percent to about 30 weight percent.
 6. The laser fiber of claim3, comprising: Thulium oxide from about 4 weight percent to about 7weight percent; Holmium oxide from about 0.3 weight percent to about 12weight percent.
 7. The laser fiber of claim 6, comprising: SiO₂ fromabout 50 weight percent to about 70 weight percent; said glass networkmodifier from about 15 weight percent to about 35 weight percent; saidglass network intermediator from about 1.5 weight percent to about 20weight percent.